Wednesday, March 11, 2009

Machine won't go into Auto Mode

I had a client call today that had a Bethel laminate press that they had moved from one factory to it's current location. They had everything back together, and it would work fine if you manually controlled it, but would not go to automatic mode when the operator told it to.

It's controlled by an Allen Bradley SLC 5/03 PLC. The first thing I thought of was a safety interlock connection was not being made.

You see, usually when an Automatic Mode is available, there are checks that the PLC makes to be certain that everything is up and running correctly before it will take full control.

So I fired up RSLogix500 and connected to the PLC. I downloaded a copy of the program off the PLC to the laptop and went online so I could monitor it as it was running.

I told the operator to press the 'auto mode' button so I could see where that input was coming into the PLC, and when I located it, I was also able to isolate the required items needed for it to go into auto mode.

I could immidiately tell that one of the needed inputs never lit up. A quick look at the machine schematics told me that that particular input was for the "Cycle Start" pushbutton. I asked the operator to toggle it so I could see if it did actually trigger the input, and low and behold, it did NOT. hmmm. Wiring problem, I bet.

So I traced the wires from the button to the PLC input module and noticed that when they had re-wired the cabinet during the re-installation, they had broken the connection from the button to the PLC. the wire was hooked up to the wrong point on a distrobution block and couldn't make the connection to the PLC.

So I moved the wire. the cycle start pushbutton started to work, so we tested the auto mode.

It worked like a charm.

The maintinance staff had worked on it for days, and didn't notice the miswired connection. They didn't have the tools to go online with the PLC and watch it operate in realtime, so they couldn't tell what was going wrong.